Observing collisions beyond the secular approximation limit

TL;DR

This paper experimentally observes nonsecular quantum dynamics in molecular collisions, revealing long-lived coherences beyond traditional approximations, using laser-kicked molecular rotors to probe rotational relaxation.

Contribution

It provides the first experimental evidence of nonsecular dynamics in molecular collisions through rotational alignment echoes, advancing understanding beyond the secular approximation.

Findings

Observation of rotational alignment echoes in gas-phase molecules

Demonstration of nonsecular quantum master equations explaining decoherence

Experimental validation of long-lived quantum coherences

Abstract

Energy transfer through quantum coherences plays an essential role in diverse natural phenomena and technological applications, such as human vision, light-harvesting complexes, quantum heat engines, and quantum information and computing. The understanding of the long-lived coherence involved in these phenomena requires a detailed modeling of the system-bath interactions beyond the so-called secular and/or Markovian approximations. Despite continuous theoretical progress on understanding nonsecular dynamics in the last decades, convincing experimental observations are still lacking. By using the laser-kicked molecular rotor as a model system, we here experimentally unveil the nonsecular dynamics in the rotational relaxation of molecules due to thermal collisions. Specifically, the rotational coherence in gas-phase molecules is systematically probed and characterized by the recently discovered rotational alignment echoes featuring a decoherence and dissipation process which can only be explained by the nonsecular quantum master equations for modeling molecular collisions.